Molybdenum trioxide layered compounds

ABSTRACT

A new composition of matter comprising molybdenum trioxide and heterocyclic nitrogen or oxygen Lewis bases. The molybdenum trioxide, upon reaction with Lewis base, forms a layered compound characterized in that the Lewis base is covalently bonded to molybdenum atoms in molybdenum oxide layers. The layered compound has the formula LMoO 3  where L is a Lewis base containing nitrogen or oxygen electron donors and selected from the group consisting of 5- and 6-membered heterocyclic amines, amine oxides, triorganophosphates, phosphine oxides and sulfoxides. L has the steric requirement such that its cross-sectional area perpendicular to an axis running through the L-Mo covalent bond is less than about 30(A) 2 .

BACKGROUND OF THE INVENTION

This invention relates to unique layered compounds formed by reacting molybdenum trioxide with a Lewis base. More particularly, Lewis bases are covalently bound to molybdenum atoms within a layered molybdenum oxide structure.

Transition metal complexes of molybdenum are well known, e.g., [Mo(NCS)₆ ]³⁻ and [Mo(2,2'-bipyridine)₃ ]³⁺. U.S. Pat. Nos. 3,489,775 and 3,153,792 describe molybdate salts formed from the reaction of molybdic acid, MoO₃ or molybdenum salts with organic nitrogenous bases. U.S. Pat. No. 4,009,122 teaches a molybdenum catalyst which is the reaction product of an oxygen containing molybdenum compound, an amine and an alkylene glycol. U.S. Pat. No. 4,010,217 relates to coordination complexes of molybdenum or tungsten with nitric oxide.

It is also known that molybdenum dichalcogenides can form intercalation compounds. Intercalation compounds wherein an organic isonitrile is intercalated into the layered structure of Group IVb, Vb, molybdenum and tungsten transition metal dichalcogenides where the chalcogenide is sulfur, selenium or tellurium are taught in U.S. Pat. No. 4,094,893. The general properties and methods of preparation of intercalation compounds are described in U.S. Pat. Nos. 3,766,064 and 3,688,109. As set forth therein, the intercalate occupies vacant sites between the layers of the metal chalcogenide wherein the chalcogenide is sulfur, selenium or tellurium. The intercalated species include organic and inorganic compounds which are broadly electron donors, electron acceptors, have substantial polarization interactions or are capable of d-orbital bonding. U.S. Pat. No. 4,049,887 relates to an improved cathode containing as active material a layered compound of the formula MA_(x) B_(y) where M is Fe, V, Ti, Cr or In, A is O, S, Se or Te, and B is Cl, Br or I.

J. Bernard and M. Camelot, C. R. Acad. Sci., Paris, Ser. C., 263:1068 (1966) report on the reaction of molybdenum trioxide, molybdenyl chloride and molybdenum dioxydichloride with pyridine. The products were identified as addition compounds of the formulae: C₅ H₅ N.MoO₃, (C₅ H₅ N)₄.MoO₃.2HCl and (C₅ H₅ N)₂.MoO₂ Cl₂. The authors noted that C₅ H₅ N.MoO₃ could only be prepared in sealed ampoules at high temperatures. In a subsequent work, M. Camelot, Revue de Chimie Minerale, 6, 853 (1969), studied addition compounds of pyridine with some oxychlorides or trioxides of chromium, molybdenum and uranium. Based on an infrared spectroscopic investigation, Camelot concluded that C₅ H₅ N.MoO₃ was a molecular coordination compound.

The preparative method described by Camelot or Bernard and Camelot for MoO₃.C₅ H₅ N is similar to a preparative technique for heavy metal chalcogenides intercalated with organic nitrogen compounds described in U.S. Pat. Nos. 3,766,064 or 3,688,109. Molybdenum, however, is not listed as a metal capable of forming a heavy metal layered chalcogenide with, e.g., pyridine. As noted above, Camelot concluded, based on his observations, that MoO₃.C₅ H₅ N was simply another molecular coordination compound similar to CrO₃.C₅ H₅ N and C₅ H₅ N.SO₃.

SUMMARY OF THE INVENTION

It has been discovered that molybdenum trioxide and Lewis bases can form new compounds having a unique layered structure. The present composition of matter comprises layered compounds of the formula LMoO₃ where L is a Lewis base containing nitrogen or oxygen electron donors and selected from the group consisting of 5-membered heterocyclic amines, 6-membered heterocyclic amines, amine oxides, triorganophosphates, phosphine oxides and sulfoxides with the proviso that the nitrogen donor cannot be unsubstituted pyridine, the layered compounds being characterized in that L is covalently bound to a molybdenum atom in the molybdenum oxide layer and L has the steric requirement such that the cross-sectional area of L perpendicular to an axis running through the L-Mo covalent bond is less than about 30 (A)².

In compounds of the invention, a neutral Lewis base is strongly coordinated to a molybdenum atom in the manner of a molecular coordination complex. In contrast to the description by Camelot in Revue de Chimie Minerale, 6, 853 (1969) for C₅ H₅ N.MoO₃, however, the present compounds are not molecular coordination complexes such as MoO₂ Cl₂.2C₅ H₅ N or (dien)MoO₃ wherein dien is diethylenetriamine (Inorg. Chem. 3, 397 (1964)) because the present compounds have an infinitely connected 2-dimensional layered structure. Moreover, the present compounds possessing a layered structure are distinguished from intercalation compounds in that their layered structure is distinct from the layered structure of the starting MoO₃.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the bonding arrangement of (4-methylpyridine)MoO₃.

FIG. 2 is a schematic diagram of the 001 projection of (4-methylpyridine)MoO₃ showing the van der Waals packing of the 4-methylpyridine molecules.

FIG. 3 is a schematic diagram of the 110 projection of the structure of (4-methylpyridine)MoO₃.

FIG. 4 is a schematic diagram of the bonding arrangement of (4,4'-bipyridine)_(1/2) MoO₃.

DETAILED DESCRIPTION OF THE INVENTION

Lewis bases which form the layered compounds of the invention are those which have heterocyclic nitrogen donors and oxygen donors, preferably 5- and 6-membered heterocyclic amine donors and especially 6-membered heterocyclic amine donors. Preferred nitrogen donors are substituted pyridines having the formula ##STR1## where R is halogen; C₁ -C₂₀, preferably C₁ -C₁₀ aliphatic; C₆ -C₁₀ aryl, preferably phenyl which may be substituted in the 4-position by halogen or C₁ -C₆ alkyl; C₇ -C₂₀ aralkyl; OR' or SR' where R' is C₁ -C₆ alkyl. Examples are ##STR2## Other 5- and 6-membered heterocyclic amines which may form layered compounds include pyridazine, pyrimidine, pyrazine, triazine, N-substituted oxazine, N-substituted imidazole, oxazole and thiazole.

A Lewis base which is a bidentate heterocyclic nitrogen ligand forms compounds of the formula L_(1/2) MoO₃, preferably compounds of the formulae ##STR3## =1,4-diaza[2.2.2]bicyclooctane) where n is 0 or 1 and R² is methylene, alkylene, alkene or alkyne of 2-6 carbon atoms; C₆ -C₁₀ arylene, preferably paraphenylene; C₇ -C₁₄ aralkylene, preferably ##STR4## where p is from 1-6; oxygen or sulfur. The bidentate ligands form connecting linkages between separate molybdenum oxide layers.

Oxygen donors may be amine oxides, triorganophosphates, phosphine oxides or sulfoxides.

Unlike the heavy metal chalcogenides employed in intercalation compounds such as those reported in U.S. Pat. No. 3,766,064, molybdenum trioxide may be used in the present invention without any special pretreatment other than drying. Thus, commercially available dried molybdenum trioxide is heated with an appropriate Lewis base in a sealed tube at temperatures of from 100° to 400° C., preferably 150° to 250° C. for up to 60 days, preferably from about 1-25 days. The amount of Lewis base is not critical and a stoichiometric amount or an excess may be used. It is preferred to evacuate the tube prior to sealing to minimize the possibility of oxidation of Lewis bases at elevated temperature. The rate of reaction may be accelerated if the Lewis base and MoO₃ are preground in a micronizing mill. An inert organic solvent may be present if desired. At higher temperatures such as 250° C., it is possible that the metal oxide itself can lead to oxidation of Lewis base. It also is preferred to use substantially anhydrous Lewis bases. Alternatively, layered compounds may be prepared by contacting an ionic molybdate salt with the protonated Lewis base as cation in the presence of Lewis base and molecular sieves. A reaction is shown as follows: ##STR5## Water produced by the reaction is taken up by the molecular sieves.

The preparation of a compound pyridine.MoO₃ has been reported (C.R. Acad. Sci., Paris, Ser. C., 263:1068 (1966)). This compound was later studied in detail (Revue de Chimie Minerale, 6, 853 (1969)). The latter spectroscopic study investigated both the uncoordinated and coordinated spectrum of pyridine as well as changes in skeletal vibrations of the inorganic part of the compound. The author reported that CrO₃, 2C₅ H₅ N; CrO₃, C₅ H₅ N; MoO₃, C₅ H₅ N; UO₃, C₅ H₅ N; MoO₂ Cl₂, 2C₅ H₅ N; UO₂ Cl₂, 4C₅ H₅ N and U₂ O₅ Cl₂, 3C₅ H₅ N all possess the characteristic bands of pyridine coordination compounds. It is probable, however, that the characterization of C₅ H₅ N, MoO₃ as a typical coordination complex is not accurate.

The layered compounds of the present invention do possess the covalent bonding characteristic of molecular coordination compounds. They are not, however, molecular coordination compounds since they possess 2-dimensional extended lattices. Nor are they intercalation compounds such as are reported in U.S. Pat. No. 3,766,064. The latter compounds are generally the result of electrostatic interactions wherein the nitrogen atom of the intercalated guest is equidistant between the layers of the intercalation host and the arrangement of host atoms within the lattice is essentially unchanged.

Rather, the instant layered compounds possess a unique physical structure as shown in FIG. 1 which is a schematic diagram of the bonding arrangement in ##STR6## As illustrated in this figure, 4-methyl pyridine occupies space between the molybdenum oxide layers and bonds directly to a Mo atom. The overall layered structure is shown in FIGS. 2 and 3 which depict the 001 projection of the structure of ##STR7## showing van der Waals packing of the ##STR8## molecules as designated by the cross-hatching and the 110 projection of ##STR9## respectively. The nature of L determines the separation of MoO₃ layers, e.g., (pyridine)MoO₃ has an interlayer separation of 11.5 A, (4-methylpyridine).MoO₃ a separation of about 13.4 A, and (4-phenylpyridine).MoO₃ a separation of about 20.4 A. As can be seen from the figures, the layers are composed of MoO₅ L octahedra sharing corner oxygen atoms. The Lewis base is trans to the single unshared oxygen. Based on x-ray studies, it can be determined that in order for the Lewis base to bond to Mo atoms, it must meet the steric requirement such that the cross-sectional area of the Lewis base perpendicular to an axis running through the Lewis base-Mo covalent bond is less than about 30 (A)². Thus bulky Lewis bases such as triethylamine or triphenylphosphine will not form layered compounds according to the invention because of the above-mentioned structural considerations.

When a bidentate Lewis base is incorporated into the LMoO₃ structure, separate molybdenum oxide layers can be bound together. For example, when 4,4'-bipyridine is employed, each separate pyridine ring can bind to molybdenum atoms in adjacent molybdenum oxide layers thus forming a composition wherein individual layers within the overall LMoO₃ crystal structures are bound together by bridging 4,4'-bipyridine molecules. This is shown schematically in FIG. 4. These bridged layered compounds are unique in that they exhibit a higher degree of thermal stability than the corresponding unbridged compounds.

If the X-ray powder pattern analysis of the layered reaction products shows lines characteristic of the MoO₃ starting material, regrinding and continued reaction can lead to a pure LMoO₃ product. The x-ray analysis further demonstrates that the interlayer distance correlates with the size of specific Lewis bases between the layers. Thermogravimetric analysis indicates the loss of Lewis base corresponding to a 1:1 or 2:1 MoO₃ :L composition. The 2:1 ratio reflects a bridging ligand such as 4,4'-bipyridine.

The layered compounds of the invention possess unique properties. First, they have a 2-dimensionally bonded layered structure with molybdenum in its highest oxidation state. Moreover, the present products are generally green and have higher thermal stability as compared to the dark blue pyridinium molybdenum bronzes reported by Schollhorn et al., Mat. Res. Bull., 11, 83 (1976). The instant layered compounds are useful in electrochromic devices and as lithium battery cathodes.

The following examples are further illustrative of the invention.

EXAMPLE 1

The preparation of ##STR10## is illustrated as follows. A pyrex tube containing MoO₃ and excess solid 4-phenylpyridine was evacuated, sealed and heated for 8 days at 170° C. After cooling, the tube was opened and the contents isolated. Solid 4-phenylpyridine was removed from the product by washing with toluene. The sample was reground and resealed in pyrex with 4-phenylpyridine and reheated for a further 10 days. The product was isolated as described above and characterized by x-ray powder diffraction and thermogravimetric analysis. The x-ray powder data showed a well defined 001 series of lines corresponding to an interlayer separation of 20.4 A. A small amount of unreacted MoO₃ was also detected. Thermogravimetric analysis gave an observed weight loss of 47.7% compared to a theoretical weight loss of 51.9% based on a 1:1 stoichiometry.

EXAMPLE 2

The general procedure of Example 1 was repeated except that 4-methylpyridine was substituted for 4-phenylpyridine. After heating for 15 days at 175° C. a dark colored product was isolated. The x-ray powder pattern showed a well developed 001 series of lines corresponding to the formation of (4-methylpyridine)MoO₃ with an interlayer separation of 13.4 A. Unreacted MoO₃ was also detected in the powder pattern.

EXAMPLE 3

The procedure of Example 1 was repeated but in this case a solution of 4,4'-bipyridine in xylene was used and the MoO₃ was preground in a micronizing mill. A mixture of preground MoO₃ was heated with an excess of the 4,4'-bipyridine/xylene solution for 4 days at 250°. After isolation and regrinding the reaction mixture was reheated for 4 days at 250° C. Finally the product was isolated, micronized and reheated for 5 days at 250° C. Thermogravimetric analysis showed a weight loss of 35.2% which leads to the formulation ##STR11## The x-ray powder diffraction pattern showed no lines for MoO₃ and a well developed 001 series corresponding to a layer spacing of 11.4 A. Well developed mixed (hkl) diffraction lines were also observed and the entire pattern could be indexed using a body-centered tetragonal unit cell with lattice parameters a=5.26 A and c=22.8 A. The doubled c axis and the stoichiometry demonstrate that the 4,4'-bipyridine ligands crosslink the molybdenum oxygen layers. 

What is claimed is:
 1. A composition of matter comprising layered compounds containing MoO₃ and nitrogen donor Lewis bases, said layered compounds having the formula ##STR12## where R is halogen, C₁ -C₂₀ aliphatic hydrocarbon, C₆ -C₁₀ aryl which may be substituted by halogen or C₁ -C₆ alkyl, C₇ -C₂₀ aralkyl, OR' or SR' wherein R' is C₁ -C₆ alkyl, the layered compounds being characterized in that ##STR13## is covalently bound to a molybdenum atom in the molybdenum oxide layer and ##STR14## has the steric requirement such that the maximum cross-sectional area of ##STR15## perpendicular to an axis running through the ##STR16## covalent bond is less than about 30 (A)².
 2. A composition of matter comprising layered compounds containing MoO₃ and nitrogen donor Lewis bases, said layered compounds having the formula LMoO₃ where L is pyridine, pyridazine, pyrimidine, pyrazine and triazine, the layered compounds being characterized in that L is covalently bound to a molybdenum atom in the molybdenum oxide layer and L has the steric requirement such that the maximum cross-sectional area of L perpendicular to an axis running through the L-Mo covalent bond is less than about 30 (A)².
 3. The composition of claim 1 wherein the layer compound is ##STR17## 